Sorry mate, but this is purely CPU's fault. You need somewhere around 2.8-3GHz to handle all the crap that happens in the background. Graphics is not the issue, this has been tested and benchmared into oblivion since the game came out, you need more juice per core, not better graphics or RAM sorry.
I ran the game with 280GTX graphics and 2.4ghz core 2 quad, upgraded to i5 and ran the game in highest settings everything maxed out etc. no issue anywhere.
^this. your CPU is the weak link here. 64 bit OS and 8g of ram would help also, but not really that much if you dont upgrade your CPU.